It has become demanded to improving various performances, such as heat resistance, mechanical properties, chemical property, electrical property and reliability, in the electronic or electrical equipment developed in recent years, as compared to the conventional electronic or electrical equipment. Polyimide and the like have been used for a film material of the insulated wire.
Advance of electrical equipment represented by motors or transformers has been progressed resulting in size reduction and improved performance. Thus, it becomes usual in many cases that insulated wires are used in such a way that the insulated wires are processed by winding (also referred to as coil processing or bending processing) to winding wires (coils) and they are pushed into a quite small space to pack. Specifically, it is no exaggeration to say that the performance of a rotating electric machine, such as a motor, is determined by how many coils produced by coil working the insulated wires can be held in a stator slot. As a result, a mechanical stress applied to the insulated wire become large, and concern is generation of insulation failure portion due to a film defect reaching the conductor.
In this case, when an electric current passes through an insulated wire assembled into electrical equipment, the insulated wire reaches a high temperature by heat generated. It is known that, in the insulating film, a film defect becomes easy to generate by a linear expansion difference at the high temperature, or by a thermal shrinkage due to a thermal deterioration. Moreover, a mechanical stress acts on or remains in the insulated wire during winding processing and also even after winding processing, and the cracking is caused in several cases. In particular, in a case where a major stress of a recent motor or the like is given to the insulated wire, this tendency is thought to be high.
Furthermore, when a thermoplastic resin layer is formed as the outermost layer by extrusion molding, a stress acted thereon during molding remains in the film resin layer even after the extrusion molding in several cases, and the cracking caused by the above-described thermal shrinkage stress and mechanical stress is induced in several cases.
On the other hand, from the past, it has been considered that if adhesion strength between an enamel-baking layer and a conductor, and adhesion strength within the enamel baking layer are increased, a processing resistance gets higher. Therefore, an attempt to enhance this adhesion strength has been made. Examples of those in which interlayer adhesion strength has been given to the enamel-baking layer include a magnet wire described in Patent Literature 1. However, in this method, because the excessively increased interlayer adhesion strength is set in order to prevent occurrence of delamination, there is a possibility that in a case where a defect has generated in a film, the cracks occur all over the film. Further, evaluation of a relatively thin enamel was conducted in this method, and there was a concern that, when the film is thickly formed in order to satisfy a highly required partial discharge inception voltage (hereinafter, referred to as PDIV) in recent years, the film cannot stand the stress given to the outer film by the bending.
Further, in a similar way, a technique of improving a PDIV property (corona resistance) for the polyimide by increasing interlayer adhesion strength, thereby to make a film thicker is also proposed (for example, see Patent Literature 2). However, even the technique described in the Patent Literature 2, because the interlayer adhesion strength is excessively increased, a film has a construction by which, when a defect generates in the film, the cracks are easy to occur all over the film.